Abstract

This study investigated the changes in soil organic matter composition by controlling fire severity of laboratory burns on reconstructed surface soil profiles (O, A1 (0–1 cm), and A2 (1–2 cm)). Laboratory burning simulated prescribed burns that would be typical in the understorey of a ponderosa pine forest at low, moderate, and high–moderate severity levels. Soils were analysed for C, N and soil organic matter composition. Soil organic matter was fractionated into humin, humic acid, fulvic acid, soluble non-humic materials and other hydrophobic compounds. In the O horizon, low-, moderate-, and high-severity treatments consumed an increasing proportion of C and N. Carbon content of the mineral soil was unaffected by burning; however, N content of the A2 horizon decreased after the moderate- and high-severity treatments, likely as a result of N volatilisation. The proportion of non-soluble material in the O horizon increased with fire severity, whereas the proportion of humin C as total C of the A horizon decreased with fire severity. The decrease in humin was followed by an increase in the other hydrophobic compounds. The higher fire intensity experienced by the burning O horizon created recalcitrant materials while an increase in labile soil organic matter was observed in mineral soil. An increase in labile soil organic matter may cause elevated C and N mineralisation rates often seen after fire.

Acknowledgements

We thank the US Joint Fire Sciences Program for the funds to conduct this study as part of the Fire and Fire Surrogates Study and Season and Interval of Burn Study. The authors would like to thank Eric Turnblom for statistical consulting; James Reardon and Beyhan Amichev for assistance during method development stages of this study; George Scherer for field-work assistance; Dongsen Xue for his laboratory assistance; and Robert Edmonds, Ron Sletten and Dan Vogt for their comments on previous versions of this manuscript. Additionally, the paper benefited from comments by three anonymous reviewers.